The current major trend in the display market has shifted from conventional high-efficiency and high-resolution-oriented displays toward displays with vivid image qualities which aim to realize natural colors with high color purity. In this sense, organic light-emitter-based organic light emitting diode (OLED) devices have been developed rapidly and research on inorganic quantum-dot LEDs having improved color purity has been actively conducted as another alternative. However, both of the organic light-emitter and the inorganic quantum-dot light-emitter have inherent limits in terms of materials.
The conventional organic light-emitters have an advantage of high efficiency but have a drawback in that they have poor color purity due to a wide spectrum. The inorganic colloidal quantum-dot light-emitters have been known to have good color purity, but have a drawback in that their color purity may be degraded because it is difficult to uniformly control a quantum dot size due to light emission caused by a quantum size effect as emitted light is shifted toward blue. Also, the two light-emitters have a drawback in that they are very expensive. Therefore, there is a need for a novel type of organic/inorganic/hybrid light-emitter to make up for the drawbacks and keep the advantages of such organic and inorganic light-emitters.
Hybrid materials composed of organics and inorganics have both the advantages of organic materials such as low manufacturing cost, simple producing and manufacturing processes, and easily controllable optical and electrical properties and the advantages of inorganic materials such as high charge mobility and mechanical and thermal stability. Therefore, the organic/inorganic/hybrid materials have come into the spotlight in both scientific and industrial aspects.
Among these, organic/inorganic/hybrid perovskite materials (hereafter “perovskites”) among the organic/inorganic/hybrid materials has a high potential for development as a light-emitter because such a material has high color purity, its colors may be simply adjusted, and has low synthesis cost. Because the perovskite material having high color purity can have a lamellar structure in which a two-dimensional (2D) plane of an inorganic substance is interposed between 2D planes of an organic substance, and has a high difference in dielectric constant between the inorganic substance and the organic substance, electron-hole pairs (or excitons) (εorganic≈2.4, and εinorganic≈6.1) are confined to an inorganic layer. Therefore, the perovskite materials having high color purity (full width at half maximum (FWHM)≈20 nm) are formed.
As a material having a perovskite (ABX3) structure in the related art, there is an inorganic metal oxide.
Such an inorganic metal oxide is generally a compound in which cations of metals (alkali metals, alkaline earth metals, transition metals, lanthanides, etc.) having different sizes, such as Ti, Sr, Ca, Cs, Ba, Y, Gd, La, Fe, Mn, etc., are positioned at the A and B sites, oxygen anions are positioned at the X site, and the metal cations at the B site are bound to the oxygen anions at the X site in the form of a corner-sharing octahedron with 6-fold coordination. Examples of the inorganic metal oxide include SrFeO3, LaMnO3, CaFeO3, etc.
On the other hand, the halide perovskite has completely different compositions than the inorganic metal oxide perovskite material because it has an ABX3 structure in which organic ammonium (RNH3) cations are positioned at the A site and halides (Cl, Br, I) are positioned as the X site, thereby forming an organic metal halide perovskite material.
Also, the characteristics of the material vary depending on a difference in such constituent materials. Typically, because the inorganic metal oxide perovskite exhibits characteristics such as superconductivity, ferroelectricity, colossal magnetoresistance, etc., the inorganic metal oxide perovskite has been generally researched to apply it to sensors, fuel cells, memory devices, etc. For example, yttrium barium copper oxide has superconducting or insulating characteristics, depending on oxygen content.
On the other hand, because the halide perovskite can have a structure very similar to the lamellar structure in that an organic plane (or an alkali metal plane) and an inorganic plane are alternately stacked, excitons can be confined in the inorganic plane. Therefore, the halide perovskite may essentially become an ideal light-emitter that emits light with very high color purity due to an intrinsic crystal structure itself rather than the quantum size of the material.
When the perovskite includes a chromophore (generally having a conjugated structure) in which organic ammonium has a smaller band gap than that of the crystal structure of a central metal and a halogen (BX3), light is emitted from the organic ammonium. As a result, the perovskite is not suitable as a light emitting layer because it does not emit light with high color purity and has a wider full width at half maximum of 100 nm or more in an emission spectrum. Therefore, in this case, this type of perovskite is not very suitable for a high color purity light-emitter emphasized in the present invention. Therefore, to fabricate the light-emitter with high color purity, it is important for the organic ammonium to include no chromophore and emit light from an inorganic material lattice composed of a central metal-halogen element. That is, this patent has focused on the development of light-emitters with high color purity and high efficiency in which light is emitted from the inorganic material lattice. For example, Korean Patent Publication No. 10-2001-0015084 (published on Feb. 26, 2001) discloses an electroluminescence device in which a dye-containing organic-inorganic hybrid material is formed in the form of a thin film rather than particles to be used as a light emitting layer, but light is not emitted from a perovskite lattice structure.
However, although the halide perovskites may emit light at a low temperature, the perovskites has a fundamental problem in that perovskites have a small exciton binding energy and excitons in perovskites are dissociated into free charges and quenched without leading to light emission due to thermal ionization and delocalization of charge carriers at room temperature. Also, when free charges are recombined to form excitons, the excitons may be quenched by neighboring layers having high conductivity, which makes it impossible to emit light. Accordingly, it is necessary to prevent quenching of the excitons to enhance luminous efficiency and brightness of the halide perovskite LEDs.